Method for drilling subminiature through holes in a sensor substrate with a laser

ABSTRACT

The present invention is a method for drilling a subminiature through hole through a substrate. The through holes of the present invention are preferably drilled through an alumina substrate which is essentially impervious to aqueous electrolytes and blood over long periods of storage in potentially reactive environments. The holes are drilled by directing a sealed type CO 2  laser at a predetermined position located on the substrate, energizing the laser at a power of approximately 75 to 225 watts, such that the laser beam focuses on the substrate at the predetermined position, and treating the substrate after the laser has drilled the hole.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to systems for analyzing fluids, and moreparticularly to a method and apparatus for drilling subminiature throughholes in a substrate of a sensor assembly for determining partialpressures of blood gasses, concentrations of electrolytes, andhematocrit value of a fluid sample.

2. Description of Related Art

In a variety of instances it is desirable to measure the partialpressure of blood gasses in a whole blood sample, concentrations ofelectrolytes in the blood sample, and the hematocrit value of the bloodsample. For example, measuring pCO₂, pO₂, pH, Na⁺, K⁺, Ca²⁺ andhematocrit value are primary clinical indications in assessing thecondition of a medical patient. A number of different devices currentlyexist for making such measurements. Such devices are preferably veryaccurate in order to provide the most meaningful diagnostic information.In addition, in an attempt to use as little of the patient's blood aspossible in each analysis performed, the devices which are employed toanalyze a blood sample are preferably relatively small. Performing bloodanalysis using a small blood sample is important when a relatively largenumber of samples must be taken in a relatively short amount of time orif the volume of blood is limited, as in neonates. For example, patientsin intensive care require a sampling frequency of 15-20 per day forblood gas and clinical chemistry measurements, leading to a potentiallylarge loss of blood during patient assessment. In addition, by reducingthe size of the analyzer sufficiently to make the unit portable,analysis can be performed at the point of care. Also, reduced sizetypically means reduced turnaround time. Furthermore, in order to limitthe number of tests which must be performed it is desirable to gather asmuch information as possible upon completion of each test. However, sizelimitations are imposed upon the sensors that are used to measure bloodchemistry. These size limitations are in large part due to physicalgeometries of the sensors and the connections to the sensors. In a bloodanalyzer disclosed in U.S. Pat. No. 4,818,361, a sensor assembly isfabricated in an attempt to reduce the size of the blood analyzer.

The sensor assembly has a plurality of sensors formed on a front side ofa polymeric form along a flow path between an inlet and outlet port. Thefluid flow path is formed as a groove in a polymeric form. The form iseither molded or machined. FIG. 1 illustrates a cross-sectional view ofthe sensor assembly. Electrodes are formed and communicate with ameasurement flow channel 34 which is formed and which communicates witha measurement flow channel 34 which is established by the combination ofsubstrate 30 and cover plate 32. FIG. 1 illustrates a pH sensor 10 and aCO₂ sensor 20. Each sensor 10, 20 includes a wire 17 which is insertedthrough and substantially fills a hole 12 in the form 30. Upon insertingthe wire 17 through the hole 12, it is critical to ensure that the wire17 completely fills the hole 12. Gaps or cavities which form between theinner walls of the hole 12 and the wire 17 act as reservoirs in whichcontaminants which can contaminate the sensor electrode can be held. Anadhesive is used to retain the wire 17 within the hole 12. Use of suchadhesive further increases the risk that the electrode will becontaminated. The wire 17 may be friction fit within the hole 12.However, insertion of the wire 17 into a tight fitting hole is verydifficult and requires excessive labor. Furthermore, even with the useof an adhesive, there is a risk that due to differences between thecoefficient of expansion of the wire 17 and the form 30, the wire 17will delaminate from the walls of the hole 12 under varying conditionsof temperature and humidity. It will be clear that such expansion andcontraction can act as a pump, drawing traces of the analyte and/orother contaminates into the cavities between the wire 17 and the wallsof the hole 12.

The end of the wire 17 opposite the measurement flow chamber 34 iscoupled to electrical conductor 19 which serves as the electricalconnection the to pH sensor 10. Conductor 19 may be in the form of aprinted circuit conductor which lies upon the surface of form 30. At theother end of electrode wire 17 is an electrochemically active layer 15.This electrochemically active layer 15 has essentially the samecross-sectional dimension as the wire 17 and serves to electrochemicallycouple wire 17 to an electrolyte layer 13. Another layer 11 is exposedto the fluid in the measurement channel 34 and covers the electrolytelayer 13. Accordingly, the shape and dimensions of the wire 17 dictatethe dimensions of the electrochemically active layer of the sensor.Thus, the sensor dimensions are limited by the dimensions of wire stockwhich is available. In addition, the shape of the electro-chemicallyactive layer of the sensor is limited by the shape of a cross-section ofthe wire 17 (i.e., essentially limiting the electro-chemically activelayer of the sensor to a circular geometry). Still further, the use ofwire 17 to fill the hole 17 places limitations on the thickness of theform, since insertion of the wire 17 into the hole 12 becomes difficultif the wire is too short.

Furthermore, since the interface between the electrically active layeris relatively large, the wire must be of a material that is compatiblewith the electrically active layer to prevent negatively effecting theoperation of the sensor. That is, over time, the conductive material ofthe wire contaminates the material used to form the electro-chemicallyactive layer of the sensor, disrupting the electrochemicalcharacteristics of the sensor. Therefore, constraints are placed on thematerial of the wires used to fill the holes 12. In one such assembly,the active layer 15 is formed from silver chloride, and the wire isformed from silver. The remaining portions of the pH sensor 10 areformed in a shallow well 14 which is concentric about the electrode hole12. The inner layer 13 is an electrolyte layer. The CO₂ sensor 20 issimilarly constructed.

In addition to the problems noted above, several other problems existwith this type of sensor. First, the process that is used to fabricatethe assembly requires that each sensor assembly be handcrafted.Accordingly, fabrication of the sensor assembly requires a substantialamount of labor which is expensive and time consuming. The hole 12associated with each sensor within the assembly must be filled with wire12 by hand one sensor at a time. In addition to the amount of laborrequired to fill each sensor hole 12, variations in the quality of theoperation and the conditions under which each hole 12 is filled increasethe possibility that the entire assembly will operate below anacceptable performance standard due to one of the sensors exhibitingpoor performance.

Second, the form 111 is fabricated from a polymeric material that tendsto absorb some of the fluid which flows through the flow path 103. Thisabsorbed fluid has a relatively low resistance compared with the veryhigh resistance required between electrodes of the sensors 101.Accordingly, the initially high resistance which exists between theelectrodes degrades. As the resistance between the conductive material301 of each sensor degrades, the accuracy of the sensors 101 degrades aswell.

Third, the electrical interface between the assembly and electronicsexternal to the assembly is through an plurality of contacts which arefabricated on the rear surface of the form. These contacts slide againsta spring loaded mating contact in the blood analyzer. As the contacts ofthe sensor assembly slide against the mating contacts within the bloodanalyzer, the contacts of the blood analyzer are worn down. Therefore,after being inserted and removed from the blood analyzer a number oftimes, the electrical connection between the external circuits withinthe blood analyzer and the sensors within the sensor assembly will bedegraded.

In addition to these problems, the blood to be analyzed must be heatedand regulated to a known stable temperature. Heating and stabilizing thetemperature of the blood can take a substantial amount of time. Stillfurther, in many cases analysis must be performed at regular and closelyspaced intervals. Accordingly, if the heating and temperaturestabilization time is relatively long, the number of times such analysiscan be performed within a particular amount of time (i.e., turn aroundtime) can be limited to a number less than would otherwise be desirable.

Accordingly, it would be desirable to provide a sensor which remainsaccurate over a relatively long period of exposure to electrolytes andblood samples, uses a very small sample size, detects the concentrationof a number of different electrolytes and the partial pressure of anumber of blood gases all in a single analysis, and in which a bloodsample may be heated very rapidly to a known stable temperature.

SUMMARY OF THE INVENTION

The present invention is a method and apparatus for drilling asubminiature through hole in a substrate of a sensor assembly. Thesensor of the present invention may be (1) a potentiometric sensor, suchas ion selective sensors; (2) an amperometric sensor (also known aspolaragraphic sensor), such as an oxygen sensor; or (3) a planarconductrimetric sensor, such as a hematocrit sensor. The subminiaturethrough hole preferably has a diameter of approximately 0.002 to 0.006inches. Because each through hole has a small diameter, the overlyingelectrode will be essentially planar after filling the holes anddepositing the layers of material which form the electrode. Also, only asmall amount of conductive material which fills each through hole is incontact with each associated electrode. Therefore, the purity of theelectrode is not significantly altered by conductive material whichfills the through hole and which is coupled to the electrode.

The through holes are laser drilled to a very accurate diameter. Inaccordance with the preferred embodiment of the present invention inwhich a 96% alumina substrate is used, after drilling, the substrate isannealed to remove any residue which attaches to the perimeter of thethrough holes. Annealing the substrate prevents contamination of thesensor electrodes which are to be deposited over the through holes.

The sensors of the present invention have very good signal to noiseratio due the essentially impervious nature of the substrate whenexposed to moisture, and due to the short electrical path length betweenthe sensors and the external detecting and analyzing electronics withinthe blood analyzer. This short electrical path is a consequence of theuse of the through holes to allow a more direct path between each sensorand the external electronics. Thus, unamplified, low level sensoroutputs from the sensors can be used directly. The use of subminiaturethrough holes to route electrical connections from the sensors toconductors on the back side of the substrate allows the sensors to beclosely spaced on the surface of the substrate. Accordingly, arelatively large number of sensors can be formed on the surface of thesubstrate within a relatively small sample path. Thus, more informationcan be attained using less blood. Furthermore, since the substrate isrelatively thin (0.025 inches) as well as small, the resulting smallerthermal mass of the sample permits the sample to be more rapid heatedand the temperature more rapidly stabilized.

Due to the use of subminiature through holes through which electricalconnections are made to each sensor, the sensors of the presentinvention can be fabricated in very small areas, allowing a relativelylarge number of sensors to be deposited in a small flowcell.Accordingly, the size of the flowcell, and thus the volume of the sampleto be analyzed, can be significantly reduced. Reduction of the volume ofthe sample within the flowcell makes it possible to rapidly bring thesample and the sensors to a stable known temperature, thus reducing theamount of time required for analyzing the sample. Furthermore, becausethe sensors are relatively small, the number of sensors that can be usedconcurrently is increased. For example, in one embodiment of the presentinvention, sensors for pCO₂, pO₂, pH, Na⁺, K⁺, Ca²⁺ and hematocrit valueare all provided in a single relatively small sample chamber.

In addition to the fact that the sensors of the present invention arerelatively small, the use of a through hole disposed directly under eachsensor, allows the entire wiring board of the present invention to bevery compact. In the preferred embodiment of the present invention, thecontact associated with each sensor on the non-sensor side of thesubstrate is in a geometric pattern which aligns the contact with aconventional surface mount electrical connector. The geometry of thesensors and the generally short conductors between the sensors and theconnector result in short conduction paths to the signal processingelectronics, and thus in a good signal-to-noise ratio undistorted byelectromagnetic radiation interference, in spite of the low signal leveloutput from the electrodes.

Still further, the use of the subminiature through holes allows thesensor assembly to be fabricated in a thick film process whichessentially completely fills the through holes in order to reduce anycontamination which might result from gaps or reservoirs forming betweenthe metal conductor and the walls of the through holes. Furthermore, thethick film process can be automated to reduce the amount of laborrequired and the variability of the sensors. Even further, the thickfilm process decreases the possibility of the metal conductor within thethrough holes delaminating, even without the need for potentiallycontaminating adhesive. In addition, the use of the subminiature throughhole of the present invention, allows exposed conductors connected tothe sensors to be spaced more widely apart, thus increasing theelectrical resistance between each such conductor.

BRIEF DESCRIPTION OF THE DRAWING

The objects, advantages, and features of this invention will becomereadily apparent in view of the following description, when read inconjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a prior art sensor assembly.

FIG. 2 is a front plan view of the sensor assembly of the presentinvention.

FIG. 3 is a back plan view of the sensor assembly of the presentinvention shown in FIG. 2.

FIG. 4 is an illustration of one pattern to which a heater conforms whendeposited on a substrate in accordance with the present invention.

FIG. 5 is an illustration of the back side of a substrate after each ofthe dielectric layers have been deposited in accordance with oneembodiment of the present invention.

FIG. 6 is an illustration of the artwork used to generate a screen,which in turn is used in the preferred embodiment of the presentinvention to deposit the second layer of conductors and connector pads.

FIG. 7 is an illustration of an oxygen sensor in accordance with thepreferred embodiment of the present invention.

FIG. 8 is a cross-sectional view of a portion of a substrate throughwhich a sensor through hole is formed and on which metal layers of anelectrolyte sensor electrode have been deposited in accordance with oneembodiment of the present invention.

FIG. 9 is a cross-sectional view of one of the hematocrit sensorelectrodes in accordance with one embodiment of the present invention.

FIG. 10 is a cross-sectional view of a sensor showing the first layer ofencapsulant in accordance with one embodiment of the present invention.

FIG. 11 is a cross-sectional view of one of the hematocrit sensorsshowing the first layer of encapsulant in accordance with one embodimentof the present invention.

FIG. 12 is a top plan view of the sensor assembly installed within aplastic encasement.

FIG. 13 is a cross-sectional view of the sensor assembly installed inthe plastic encasement.

FIG. 14a-14c illustrate alternative embodiments of the present inventionin which the relative positions of the sensors differ from those shownin FIG. 2.

FIG. 15 is a flow chart of the steps of the present invention.

LIKE REFERENCE NUMBERS AND DESIGNATIONS IN THE VARIOUS DRAWINGS REFER TOLIKE ELEMENTS. DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the preferred embodiment and examples shownshould be considered as exemplars, rather than limitations on thepresent invention.

Overview

FIG. 2 is a front plan view of one embodiment of the sensor assembly 400of the present invention. FIG. 3 is a back plan view of the sensorassembly 400 of the present invention shown in FIG. 2. The presentinvention is a sensor assembly 400 having a plurality of sensors 403,including highly pure, planar circular silver potentiometric andamperometric electrode sensors disposed on an inorganic substrate 405.The sensor assembly 400 is preferably enclosed within a housing whichdefines a flowcell into which an analyte is transferred for analysis bythe sensors 403. Each sensor 403 is fabricated over a subminiaturethrough hole through the substrate 405. In accordance with the preferredembodiment of the present invention, each subminiature through hole ispreferably laser drilled through the substrate. These through holesreduce the amount of area required on the front side of the substrate byeach of the sensors 403. That is, the present design geometry permits anumber of sensors to be arrayed in a plane with fewer restrictions,since the layers of the conductors do not interfere with the placementof the sensor electrodes. Reducing the required area on the front sideof the substrate allows a relatively large number of sensors 403 to belocated in a relatively small area on the sensor assembly 400, and thusallows the volume of the flowcell to be reduced. Reducing the volume ofthe flowcell reduces the sample size, which is important, since in somesituations many samples are required from the same patient. Furthermore,as a consequence of the small sample size, the low thermal mass of thesensor assembly 400, and the placement of a heater on the back side ofthe substrate, the present invention rapidly reaches a stabletemperature at which analysis can be performed. Accordingly, the presentinvention can be installed into a blood analyzer (not shown) to providerapid results (i.e., approximately 60 seconds in the case of oneembodiment).

In addition to reducing the area required for each sensor 403, the useof subminiature through holes through the substrate under each sensor403 allows the sample and reference solution to be physically isolatedby the substrate 405 from the electrical conductors 410 which transferelectrical charge or current from each sensor electrode to an associatedconnector pad 411 (see FIG. 3). Only the sensor electrodes and athermistor 409 are located on the front side of the substrate. Thepredominant use of the back side of the substrate to route conductorsallows the front side of the substrate (i.e., where surface area is at amuch greater premium) to be reserved for those elements which mustreside on the front side (such as the sensor electrodes). It should benoted that the conductors 410 and pads 411 are shown using broken linesin FIG. 3 to illustrate that an encapsulant 415 is applied over theconductors 410 and a portion of the pads 411. As will be discussed ingreater detail below, solder is deposited over the pads 411 to providean appropriate electrical and physical interface to a surface mountconnector (not shown in FIG. 3). As will also be described in moredetail below, the thermistor 409 (see FIG. 2) is also encapsulated afterbeing deposited on the front of the substrate 405. While the term"deposited" is used throughout this document, the meaning is intended tobe inclusive of all means for forming a structure in a layered device,including screening, plating, thick film techniques, thin filmtechniques, pressurized laminating, photolithographic etching, etc.

In accordance with one embodiment of the present invention, all of theconnections which couple the sensors 403 to external devices aredeposited on the back side of the substrate. These connections arespaced apart to provide the greatest possible insulation resistance. Inone embodiment of the present invention, electrical conductors aredeposited on a plurality of different fabrication layers deposited onthe back side of the substrate 405. No sample or reference solutioncontacts the back side of the substrate, as will be clear from thedescription provided below. A conventional surface mount electricalconnector is preferably mounted on the connector pads to provide anelectrical conduction path through a mechanical interface from thesensors 403 to external devices which detect and process the electricalsignals generated by the sensors 403.

The substrate 405 of the preferred embodiment of the present inventionis essentially impervious to aqueous electrolytes and blood overrelatively long periods of time (i.e., more than six months in the caseof one embodiment of the present invention). In accordance with thepreferred embodiment of the present invention, the inorganic substrate405 is a sheet of approximately 0.025 inch thick commercial grade 96%alumina (Al₂ O₃). The substrate 405 is preferably stabilized by a heattreatment prior to purchase. One such substrate is available from CoorsCeramic Company, Grand Junction, Colo. Alternatively, the substrate maybe either a more or a less pure ceramic substrate. Furthermore, thesubstrate may be any non-conductive essentially flat surface upon whichthe sensors may be deposited, as will be described in further detailbelow. For example, the substrate may be any silicon, glass, ceramic,wood product, non-conducting polymer or commercially available frit thatcan be used as a substantially smooth flat surface. However, thesubstrate preferably should be capable of withstanding the presence ofan electrolyte having a pH of more than 6 to 9 and remaining essentiallyunaffected for an extended period of time (i.e., in the order of weeks).

Use of an alumina substrate provides the following advantages: (1) lowthermal mass; (2) dimensional stability when subjected to aqueouselectrolytes and blood for extended periods time; (3) establishes amechanically and chemically stable substrate for use with thick filmdeposition techniques; (4) can be accurately laser drilled to highprecision with very small diameter holes; (5) does not react with any ofthe materials which are used to fabricate sensors; and (6) very highelectrical resistance. As a consequence of the fact that the assembly,including the inorganic substrate 405 and each deposited layer, is verystable and does not breakdown when subjected to aqueous electrolytes andblood, the sensor assembly 400 maintains very high isolation between (1)each of the sensors 403; (2) each of the sensors 403 and each electricalconductor; and (3) each of the electrical conductors.

Because the substrate 405 and each of the layers deposited thereon arestable and resists breakdown in the presence of aqueous electrolytes andblood, extremely high electrical resistance is maintained between eachconductor coupled to the sensors. Accordingly, the present inventionprovides very high electrical isolation between each of the sensors 403,even after exposure to a reactive environment over a relatively longperiod of time. This is advantageous because it is desirable to storethe sensor assembly of the present invention in a relatively reactiveenvironment. That is, the present sensor assembly 400 is preferablystored in a sealed pouch (not shown) having a humidity that reducesevaporation of the isotonic reference medium. Storing the presentinvention in a sealed pouch having a controlled humidity also ensuresthat the sensors 403 remain partially hydrated during storage. Since thesensors 403 remain partially hydrated during storage of the sensorassembly 400, the sensors 403 of the present invention require minimalconditioning after installation. Therefore, having the sensors 403stored in partially hydrated state greatly reduces the amount of timethe user must wait before results can be attained from the sensors 403of the present invention. This differs from prior art pH and CO₂ sensorswhich are stored in an essentially dry environment. Such prior artsensors must be assembled or preconditioned many hours prior to use. Itis advantageous to provide a sensor assembly 400 which is available foruse shortly after installation. For example, blood laboratories whichuse prior art blood analyzers must maintain at least two such prior artblood analyzers or risk being out of service for many hours afterreplacement of a sensor assembly (i.e., the time required to assemble,condition, calibrate, and rehydrate the sensors). The sensor assembly ofthe present invention can output results in as little as 10 minutes fromthe time the sensor assembly is installed, thus reducing the need for asecond blood analyzer which would otherwise be required as a backup.

In accordance with one embodiment of the present invention, anisotonicreference medium (e.g., a gel or other a viscous solution having a knownion concentration) is placed over a reference electrode to provide areference for potentiometric sensors which are fabricated on thesubstrate 405. In accordance with the sensor assembly 400 shown in FIG.2 and 3 the following sensors are provided: (1) sodium sensor 403h; (2)potassium sensor 403g; (3) calcium sensor 403f; (4) pH sensor 403e; (5)carbon dioxide sensor 403a; (6) oxygen sensor 403b; and (7) hematocritvalue sensor 403c, 403d. A reference electrode 407 is also provided. Thereference electrode is common to each of the potentiometric sensors(i.e., the sodium sensor 403h, potassium sensor 403g, calcium sensor403f, pH sensor 403e, and carbon dioxide sensor 403a) and provides avoltage reference with respect to each such sensor. It will beunderstood by those skilled in the art that these sensors, or any subsetof these sensors, may be provided in combination with other types ofsensors.

Fabrication of the Sensor Assembly of the Present Invention

The following is the procedure by which one embodiment of the presentinvention is fabricated. It will be understood by those of ordinaryskill in the art, that there are many alternative methods forfabricating the present invention. Accordingly, the description of thepreferred method is merely provided as an exemplar of the presentinvention.

Initially, a series of through holes are drilled through the substrate405. The substrate is preferably 0.025 in. thick. Alternatively, thelaser technique described below may be used for a substrate that is asmuch as 0.040 in. thick. Preferably, each through hole is laser drilledusing a conventional CO₂ laser (such as a Diamond 64 laser availablefrom Coherent, Inc., Laser Group, Santa Clara, Calif.) to a diameter inthe range of approximately 0.002-0.006 inches, as measured on the frontside of the substrate 405. In an alternative embodiment of the presentinvention, any conventional laser which is capable of drilling throughthe substrate with less than a 0.006 diameter may be used. For example,a neodymium doped yttrium-aluminum-garnet (Nd:Yag), or excimer laser maybe used. However, such lasers drill at lower power, and therefore areslower.

The laser beam of the preferred CO₂ laser has a wavelength ofapproximately 10.6 μm. This wavelength is readily absorbed by aluminaceramic substrates. However, it will be understood by those skilled inthe art that other wavelengths may be more appropriate to othersubstrate materials. The beam is controlled to focus within a relativelysmall diameter. In the preferred embodiment of the present invention,the laser beam is focussed to a spot size of between 0.001 and 0.003inches. Due to the interaction between the substrate 405 and the laserbeam, the through hole that is formed is tapered (i.e., the diameter ofthe resulting through hole is greater at the side of entry of the laserthan at the side of exit). In the preferred embodiment, the smalldiameter opening is formed on the front side of the substrate 405.

FIG. 15 is a flow chart of the steps of the present invention.

The laser is directed at each location on the substrate at which a holeis to be formed. Preferably, a thin water soluble coating is applied tothe substrate before drilling. For example, in accordance with oneembodiment of the present invention, the substrate is dipped in a lowviscosity aqueous solution of acrylic resin or polyvinyl alcohol anddried. Such a coating allows much of the micro-drilling swarf thatcondenses on the substrate to be removed in flowing water upon lightlybrushing the substrate. Such a coating also reduces "kerf" (a build upof debris or "slag" around the hole). The laser is pulsed at 1000 Hz in38 μsec length pulses (equivalent to a rate of 0.25 sec./hole) with thebeam focussed on the surface of the substrate at the point at which thethrough hole is to be formed. Pulsing the laser is preferred to reducethe heat lost to the substrate while ablating and vaporizing thematerial. Air is preferably blown across the location of the substrate405 at which the laser is focused preferably at a pressure of at leastabout 20 pounds per square inch to displace debris generated by theinteraction between the substrate 405 and the laser. The forced airremoves a substantial portion of the liquid created by the beam as suchliquid is created, thus the beam need not volatize the entire volume ofmaterial to be removed. The result is a hole of average diameter ofapproximately 0.0023 in. as measured optically at the beam exit side.

In one alternative embodiment of the present invention, through holesare drilled with a flow through CO₂ laser, such as a 150 watt to 10kilowatt flow through type CO₂ laser manufactured by Lumonics, Ontario,Canada. Alternatively, the laser may be a sealed type CO₂ laseroperating in a power range of 75 to 225 Watts. Use of such a laserresults in through holes with an average diameter of approximately0.0035 in. at the side of the substrate from which the beam exits. Aquench zone of damage around the inner diameter of the resulting throughhole results from the glassy phase build-up typical of the laserdrilling process.

In accordance with the preferred embodiment of the present invention,the substrate is placed on a conventional high accuracy X-Y motioncontrol platform with positional accuracy of better than +/- 0.001 in.Very complex hole patterns may be micro-drilled by entering positionalcoordinates in the conventional motion control software associated withthe motion control platform. Because the through holes are relativelysmall and the power density of the CO₂ laser is relatively great, theholes are preferably drilled at a rate of 250-1200 holes/minute.

The interaction between the laser beam and the substrate 405 leaves areactive noncrystalline, nonstoichiometric, thermal shock microcrackedsurface within the hole after drilling. In addition, some plasmafied orvolatilized material hardens and reforms on the walls of the hole.Therefore, when the substrate 405 is a ceramic material, such asalumina, or a silicon based material, the substrate is preferablyannealed after drilling all of the through holes. Annealing thesubstrate reoxidizes and recrystallizes the glassy surface layer of thehole to more closely resemble its undrilled state. Accordingly, thethick film metals that are preferably used to fill the subminiaturethrough holes will react normally even at the heat affected interface.Thus, annealing prevents contamination of the electrodes which are to beformed over the subminiature through holes. That is, in the case inwhich an alumina substrate is not annealed, a subsequently formed silverpotentiometric electrode will develop contamination at the periphery ofthe through hole upon firing of the silver electrode. Such contaminationdegrades the performance of a potentiometric sensor formed by theelectrode. Furthermore, the adhesion to the walls of the through hole bythe thick film metallization that fills the through hole would bedegraded without the annealing process. The problem of adhesion may beparticularly acute in the case of pure silver electrodes. Annealingrestores the surface of the substrate to its native condition to providea suitable surface upon which a wide range of thick film metallizationswill adhere. In particular silver metallizations which are preferred foruse in potentiometric sensor electrodes adhere well to 96% aluminasubstrates which have been drilled and subsequently annealed. Annealingthe substrate after drilling ensures re-oxidation of a nonstoichiometricresidue that attaches to the holes after the laser drilling. Withoutannealing, the residue (which is very reactive) contaminates the sensorelectrodes, resulting in less pure electrode surfaces, which can lead topoor sensor performance.

In accordance with one embodiment of the present invention, thesubstrate is annealed by baking at a temperature in the range ofapproximately 1000°-1400° C. for a period of 1-3 hours, and morepreferably in the range of approximately 1100°-1200° C. for a period ofthree hours. The substrate is preferably placed on a ground flat ceramicplate. A flat weight (preferably having a ceramic surface in contactwith the substrate) weighing several pounds is laid upon the substrate,thus sandwiching the substrate between the oven floor and the weight toprevent warping of the substrate during the annealing process. Thetemperature is preferably increased and decreased at a rate ofapproximately 1° C./minute. Alternatively, the temperature may beincreased and decreased at a higher rate, such as 3° C./minute uponensuring that no stressing of the substrate occurs at such higher rates.Stressing of the substrate is, at least to some degree, related to anyscribed patterns on the substrate. Accordingly, in the preferredembodiment, any scribing of the substrate is performed after thesubstrate has been drilled and annealed. However, in an alternativeembodiment of the present invention, the substrate may be scribed eitherbefore annealing, or not at all. Scribing the substrate allows severalindividual sensor assemblies formed in the same deposition processes onone substrate to be separated after all of the assemblies have beencompleted.

In an alternative embodiment of the present invention, the substrate 405is treated by subjecting the substrate to a caustic solution, such ashydrofluoric acid. In accordance with one such embodiment, a solution of3-35% hydrofluoric acid may be used to anneal the substrate 405. Thesolution is preferably warmed to a temperature of approximately 40° C.to increase the rate at which the solution reacts with the substrate405.

By maintaining the small diameter of each through hole, the planarcharacteristic of an electrode which is deposited over the through holeis not distorted by the presence of the through holes. In the preferredembodiment of the present invention, thirteen holes are required, suchthat one hole is provided for each sensor, except for the hematocritsensor 403c, 403d and the oxygen sensor 403b, each of which require twoholes. The hematocrit sensor requires two holes in light of the twoelectrodes 403c, 403d. The oxygen sensor 403b preferably has one throughhole for connection to the cathode of the sensor and one through holefor connection to the anode of the sensor. In addition, two throughholes are preferably used for the connections to the thermistor 409.Also, two through holes are preferably used for the reference electrode407 to reduce the risk of a defective through hole creating an opencircuit. In the preferred embodiment of the present invention, eachthrough hole that is associated with a sensor electrode is located underthe location at which the associated sensor electrode to be deposited.Each such through hole is preferably located essentially at the centerof the sensor electrode with the exception of the oxygen sensor 403b.However, in an alternative embodiment of the present invention, eachthrough hole may be located anywhere underneath an electrode.

Once the through holes have been drilled and annealed, a thermistorpaste is deposited in a predetermined pattern on the front side of thesubstrate 405 to form a thermistor 409 as shown in FIG. 2. In analternative embodiment of the present invention, the particular geometryof the thermistor may vary from that shown in FIG. 2. In an alternativeembodiment, the thermistor 409 is a discrete component which is notformed directly on the substrate. In the preferred embodiment of thepresent invention, the thermistor paste is part number ESL 2414,available from Electro-Science Laboratories, Inc. The thermistor paste501 is preferably deposited to a thickness of approximately 15-29 μMwhen dried (10-22 μM when fired). The thermistor paste is oven dried andfired at a temperature of approximately 800°-1000° C. for approximately1-20 minutes. It will be understood by those skilled in the art that thethermistor 409 may be fabricated with any material that will provideinformation to an external control device by which the temperature ofthe sensor assembly 400 can be controlled. The thermistor is preferablybe placed adjacent to any sensor that is particularly temperaturesensitive or appropriately when measuring a temperature sensitiveanalyte. In an alternative embodiment of the present invention, a numberof sensors and independently controllable heaters may be used toregulate the temperature of each sensor and the local temperature of theanalyte at different locations along the flow path.

Once the thermistor paste has been deposited, dried, and fired, thesubstrate 405 is preferably placed in a vacuum fixture. The vacuumfixture (not shown) has a plurality of vacuum ports, each placed incontact with the opening of a through hole on the front side of thesubstrate. Preferably, each vacuum port is concurrently aligned with oneor more of the through holes to create a relative low pressure withineach through hole of the substrate with respect to the ambient pressureoutside the through holes. A thick film metallic paste, which ispreferably compatible with the metal to be used to form the metalliclayer of the electrodes of the electrolyte sensors 403h, 403g, 403f, aswill be described in more detail below, is deposited over the throughholes on the back side of the substrate 405. Such thick film metalpastes are available with metallic phases of pure silver, gold, andplatinum. The choice as to which of these is to be used in each throughhole depends upon the type of sensor electrode with which the throughhole is associated and the metal which is used with in that electrode.In an alternative embodiment of the present invention, binary or ternaryalloys of silver and noble metal pastes are used. Alternatively,solution plated or electroplated plated metals or reactive crosslinkableliquid polymers-metal formulations may be used.

The deposited metal forms a conductive pad over the through hole.However, due to the vacuum applied to the front side of the substrate405, a portion of the metal is drawn through the through holes. Inaccordance with the present invention, the metallic paste is preferablya silver paste, such as part number ESL 9912F, available fromElectro-Science Laboratories, Inc. In accordance with the preferredembodiment of the present invention, the metallic paste is appliedthrough a screen having a mesh density of 250 wires per inch (each wirehaving a diameter of approximately 0.0016 inches and a spacing of 0.0025inches) and an emulsion thickness of approximately 0.0007 inches. Theemulsion is developed to form a mask which allows the metal paste topass through the screen only at the locations of the through holes onthe back side of the substrate 405. The metallic paste is formed by thescreen into columns above each through hole. Those columns of metalpaste are then drawn down into the through holes by the reduction inpressure caused by the vacuum fixture. This procedure is preferablyperformed twice to ensure that each through hole is filled with thesilver paste. In accordance with the preferred embodiment of the presentinvention, the paste which is drawn into the through holes establishes areliable, chemically inert electrical interconnection between the sensorelectrodes and electronic circuits that are external to the assembly400. In yet another alternative embodiment in which the substrate issilicon based, the through holes are printed with a thick film metalpaste or metal filled resin paste which can be fired at a relatively lowtemperature (i.e., less than 600° C.).

The substrate is then rotated to place the back side of the substrate405 in contact with vacuum ports. The ports are aligned with the throughholes over which the hematocrit electrodes 403c, 403d are to bedeposited. The metal with which the front side of the through holes arefilled is preferably selected to be compatible with the particular metalfrom which the electrode to be formed over the through hole is to beformed. In the preferred embodiment of the present invention, thehematocrit electrodes are formed using platinum. Therefore, the metallicmaterial which fills the front side of these through holes and formsconductive pads on the front side of the substrate is preferably asilver/platinum paste, such as a mixture of silver paste, part number QS175, available from dupont Electronics, and platinum paste, part numberESL 5545, available from Electro-Science Laboratories, Inc. The use of asilver/platinum paste presents a compatible interface between theplatinum hematocrit sensor electrodes and the silver conductive materialwhich fills the back side of the through holes which will underlie thehematocrit sensor electrodes. The mixture preferably has 50 partssilver, and 50 parts platinum. However, in an alternative embodiment,other alloys of silver and platinum may be used. Furthermore, any alloywhich is compatible with platinum (i.e., with which platinum forms asolid solution), may be used. In a next screening process, each of theother eleven through holes (i.e., each of the through holes except thetwo over which the hematocrit electrodes 403b, 403c are to be deposited)are preferably filled from the front side of the substrate 405 using thesame metallic paste that was previously used to fill the through holesfrom the back side of the substrate. Conductive pads, similar to theconductive pads formed on the back side of the substrate 405, are formedon the front side of the substrate 405. Filling the through holes fromboth the front and the back side of the substrate ensures that theentire through hole will be filled, and that a low resistance electricalcontact will be made between the front and back side of the substratethrough each through hole.

FIG. 4 is an illustration of one pattern to which a heater 601 conformswhen deposited on the substrate 405 in accordance with the presentinvention. In the embodiment shown, the heater 601 conforms generally toa complex serpentine pattern. FIG. 4 also shows a number of electricallyconductive traces 603 which provide electrical conduction paths forcurrent and/or electrical potential to be communicated from theelectrodes of the sensors 403 to the pins of a connector to be affixedto the substrate, as will be described in greater detail below. Theheater 601 is preferably deposited on the back side of the substrate405. In accordance with one embodiment of the present invention, aheater paste blend including 10 parts of part number C4081, availablefrom Heraeus Cermalloy, and 90 parts of part number 7484 available fromDuPont Electronics is deposited to a thickness of 15-33 μM dried (7-20μM fired). In accordance with one embodiment, a through hole vacuum maybe applied to seal any through holes that remain open. It will beappreciated by those skilled in the art that the heater may be anyheater device that provides a source of heat which can be readilycontrolled by a control device that receives information regardingtemperature from the thermistor 409. It will also be appreciated thatthe particular routes taken by the conductors 603 may vary inalternative embodiments of the invention.

Once the heater 601 and conductors 603 have been deposited, a series ofdielectric layers 419 are deposited on the back side of the substrate405 which electrically insulate the heater 601 and the conductors 603from additional layers which are to be later deposited over the heater601 and the conductors 603. The dielectric includes openings throughwhich "vias" can be formed to provide electrical contact paths to theconductors 603 through the dielectric layers. A dielectric paste (suchas part number 5704, available from E.I dupont) is applied to the backside of the substrate 405, preferably using a conventional thick filmscreening technique. The screen used to apply the dielectric paste masksall locations except those at which a via is to be formed. FIG. 5 is anillustration of the back side of the substrate 405 after each of thedielectric layers 419 have been deposited. It should be noted that theheater 601 and conductors 603 are shown in broken lines to indicate thepresence of the dielectric layer 419 over the heater 601 and conductors603. After two layers of the dielectric paste have been deposited,dried, and fired at a temperature of approximately 800°-950° C., ametallic paste, such as a palladium/silver composite, which in thepreferred embodiment is part number 7484, available from E.I. dupont, isdeposited over those locations 750 at which vias are to be formed. In analternative embodiment of the present invention, other noble metalmixtures can be used to achieve the desired resistance value within theavailable surface area. The metallic paste is then fired at 800°-950° C.for approximately 1 to 20 minutes. Two more layers of dielectric pasteand metallic paste are deposited, each such layer being fired at800°-950° C. for approximately 1 to 20 minutes directly after beingdeposited. It will be clear to those skilled in the art that othermethods for depositing the dielectric layer and the vias may not requiremultiple layers of dielectric and metal. However, due to limitations onthe thickness of layers which are deposited through a screen, more thanone layer of both dielectric paste and metallic paste are preferablydeposited. The dielectric layers between the conductive lines of theheater 601 build to a height which is nearly equal to the height of thedielectric layer over the heater 601, thus providing a relatively smoothsurface at the back side of the sensor assembly 400.

After the last dielectric layer 419 is deposited, a second layer ofconductors is deposited. FIG. 6 is an illustration of a secondconductive layer, including the second layer of conductors 410, aplurality of connector pads 411, and connections 803 to the resistor 412(see FIG. 3). In one embodiment of the present invention, the secondconductive layer is formed from a metallic paste, such aspalladium/silver, which in the preferred embodiment of the presentinvention is part number 7484 available from E.I dupont. The secondconductive layer is then oven dried and fired at a temperature in therange of approximately 800°-950° C. for approximately 1 to 20 minutes.The conductors 410 and conductive connector pads 411 complete theconnection between the sensor electrodes and external devices (notshown) coupled to the connector fixed to the connector pads 411. Thesecond layer of conductors is oven dried and fired at a temperature inthe range of approximately 800°-950° C. for approximately 1 to 20minutes.

In accordance with the present invention, conductors 603, 410 aredeposited on only two layers (i.e., the heater layer and the connectorpad layer). However, in an alternative embodiment of the presentinvention in which the geometry of the sensor assembly 400 makes itdifficult to route the conductors from each sensor to an appropriateelectrical contact pad to which a connector is to be electricallycoupled, more than two layers having conductors may be used. In such anembodiment, each such conductor layer is preferably separated by atleast one layer of insulating dielectric material.

After the second layer of conductors has been deposited on the back sideof the substrate 405, each of the layers which form the electrodes ofthe sensors 403 are deposited on the front side of the substrate 405.Concurrent with the deposition of the first metal layer of eachelectrode, contacts 414 to the thermistor 409 are deposited to couplethe thermistor to the through holes that are adjacent the thermistor 409(see FIG. 2). FIG. 7 is an illustration of an oxygen sensor 403b' inaccordance with an alternative embodiment of the present invention. Boththe oxygen sensor 403b and 403b' are essentially conventionalamperometric cells. The only difference between the oxygen sensor 403bshown in FIG. 2 and the oxygen sensor 403b' shown in FIG. 7 is the shapeof the anodes 701, 701'. In accordance with the preferred embodiment ofthe present invention, the anodes 701, 701' are essentially straightconductors which deflect from straight at the distal end 703, 703'.Preferably, the area of the anode is a minimum of 50 times greater thanthe area of the cathode to ensure the most stable operation. Inaddition, the distance between the anode and the cathode is preferablyapproximately 0.020-0.030 inches to ensure that the potential developedacross the anode to cathode is not too great. It should be noted thatthe anode of the oxygen sensor may be configured to conform to anynumber of alternative shapes. These two shapes are provided merely asexemplars of the shape of the anode in accordance with two particularembodiments of the present invention. In one embodiment of the presentinvention, a metal, such as silver paste, part number QS 175, availablefrom DuPont Electronics, is deposited to form the anode 701, 701' of theoxygen sensor 403b'. Alternatively, any metal suitable for use informing the anode of an amperometric cell may be used, such as platinum,ruthenium, palladium, rhodium, iridium, gold, or silver. A distal end703, 703' of the anode 701, 701' is deposited over one of the abovedescribed through holes 705 through the substrate 403.

The cathode conductor 707 is then deposited. A distal end 709 of thecathode conductor 707 is deposited over another of the through holes 711through the substrate 403. The cathode conductor 707 and the anode 701,701' are oven dried and fired at a temperature of approximately 800° C.to 950° C. for approximately 1 to 20 minutes.

FIG. 8 is a cross-sectional view of a portion of the substrate 405through which a sensor through hole 702 is formed and on which metallayers of an ion sensitive sensor electrode have been deposited.Concurrent with the deposition of the oxygen sensor 403b, and bydeposition of the same type of material (preferably silver) deposited toform the metallic layer of the anode 701, 701' of the oxygen sensor403b, a first metallic layer 704 of each of the electrodes associatedwith each of the other sensors 403a, 403e-403h and the referenceelectrode 407 are deposited on the substrate over a through hole 702. Inthe case of sensors 403a, 403e-403h which are to have a polymericmembrane disposed over the metallic layer, a second metallic layer 706,preferably of the same material as the first metallic layer 704, isdeposited over the first metallic layer 704 in order to reduce anydistortion in the flatness of the surface due to the presence of thethrough hole 702 located beneath the first metallic layer 704. That is,electrodes formed over a through hole 702 with only one layer ofmetallic material tend to develop a depression over the through hole702. Such a depression is generally of no consequence if the electrodeis not to be coated with a polymeric membrane.

However, in sensors which have polymeric membranes, such a depressioncan cause the membrane to become embedded in the electrode 704. As aresult of this distortion, optimal performance would not be achieved.That is, very uniform membrane geometry is important to achievingoptimal sensor function and performance. This can be understood in lightof the fact that in the preferred embodiment of the present invention,the thickness of a polymeric membrane that is applied over the metalliclayers 704, 706 is determined by pouring a controlled volumetricquantity of a membrane solution into a sensor cavity having well defineddimensions (as will be discussed further below). The membrane formedover the metallic layer 706 is very thin (i.e., approximately 5-250 μM).Any variation in the thickness of the membrane at one point, effects thethickness of the membrane at each other point. Such variations in thethickness of the membrane adversely effect the performance of the sensor403. Therefore, if a depression exists in the metallic layer whichunderlies the polymeric membrane, the membrane will be thicker over thedepression, and thus thinner over the remainder of the electrode.Depositing a second metallic layer 706 smooths any such depression whichmight otherwise exist. The second metallic layer 706 preferably has adifferent diameter than the first layer 704 in order to reduce thechances that the metallic layers will puncture the polymeric membranedue to the abrupt edge that would be formed at the perimeter if both thefirst and second metallic layers 704, 706 were to have the samediameter. Since the presence of a depression is insignificant inelectrodes of sensors which do not require a thin membrane, thesesensors are preferably formed having only one metallic layer 704.

The preferred dimensions for the metallic layers 704, 706 of each sensorin accordance with one embodiment of the present invention are providedbelow. It will be understood by those skilled in the art that otherdimensions may be quite suitable for fabricating sensors. However, thedimensions presented reflect a tradeoff between reduced impedance andreduced size. A tradeoff is required because of the desire to form thesensor in as small an area as possible, and the competing desire to forma sensor which has a relatively low impedance. These two goals areincompatible because of the inverse relationship between size andimpedance. That is, in general, size is inversely proportional toimpedance. Therefore, the greater the size of the sensor electrode, thesmaller the impedance of that electrode.

The diameter of the first metallic layer 704 of the CO₂ sensor 403a, thepH sensor 403e, and each of the electrolyte sensors 403f, 403g, 403h is0.054 inches. The diameter of the second electrode layer 706 of each ofthese sensors is 0.046 inches. The second layer 706 is deposited overthe first layer 704. The metallic layer 704 of the reference electrodeis generally rectangular, having rounded comers with radius equal to onehalf the width of the electrode. The width of the electrode ispreferably 0.01 inches, and the length is preferably 0.08 inches. Itwill be understood by those skilled in the art that the referenceelectrode 407 may be formed in numerous other shapes. After the firstmetallic layer 704 is deposited, the substrate 405 is oven dried andfired at approximately 800°-950° C. for approximately 1-20 minutes.After deposition, the second metallic layer 706 is similarly dried andfired. Each of the metallic layers 704, 706 is preferably 16-36 μM thickafter drying, and 7-25 μM thick after firing.

FIG. 9 is a cross-sectional view of one of the hematocrit sensorelectrodes 403c. Only one of the two electrodes 403c, 403d are shown,since each are essentially identical. In accordance with the preferredembodiment of the present invention, the metal used to form theelectrodes of the hematocrit sensor 403c, 403d differs from the metal704, 706 used to form the electrodes of the electrolyte sensors 403f,403g, 403h, the pH sensor 403e, the oxygen sensor 403a, and thereference electrode 407. Therefore, in the preferred embodiment, theelectrodes of the hematocrit sensor 403c, 403d are formed by depositinga third metallic layer 1001. Since no polymeric membrane is to be placedover the metallic layer 1001 of the hematocrit electrodes 403c, 403d,the hematocrit electrodes 403c, 403d preferably only have one metalliclayer. In the preferred embodiment of the present invention, the metalused to form the electrodes for the hematocrit sensor 403c, 403d is acermet platinum conductor, such as part number ESL 5545, available fromElectro-Science Laboratories, Inc. The diameter of the metallic layer1001 of each hematocrit sensor electrode is 0.054 inches. The hematocritsensor electrodes 403c, 403d are preferably spaced approximately 0.15inches apart.

After forming the metallic layer 1001 of the hematocrit sensorelectrodes 403c, 403d, the cathode conductor 707 (see FIG. 7) isdeposited. In accordance with the preferred embodiment of the presentinvention, the cathode conductor 707 is formed from a gold paste, suchas part number ESL 8880H, available from Electro-Science Laboratories,Inc. It will be understood by those skilled in the art that the cathodeconductor 707 may be fabricated from any metal commonly used to form acathode of a conventional amperometric cell. However, it should be notedthat the level of contaminants in the paste will effect the sensorcharacteristics. Furthermore, in an alternative embodiment of thepresent invention, the particular geometry of the cathode conductor 707may vary from that shown in FIG. 7. At the same time that the cathodeconductor 707 is deposited, a pair of laser targets 417, 418 arepreferably deposited. The laser targets 417, 418 provide a referencewhich is used to form a cathode 717, as will be discussed in greaterdetail below. Once deposited, the cathode conductor 707 is dried andfired at a temperature of 800°-950° C. for approximately 1 to 20minutes.

Once the cathode conductor 707 has been dried and fired, a resistor 412is preferably deposited on the back side of the substrate 405, as shownin FIG. 3. The resistor 412 is coupled in series with the heater 601 inorder restrict the current to an appropriate level through the heaterduring electrical conduction. Next, a first layer of an encapsulant isdeposited on the front side of the substrate 405. FIG. 10 is across-sectional view of a sensor 403 showing the first layer ofencapsulant 901. FIG. 11 is a cross-sectional view of one of thehematocrit sensors 403c showing the first layer of encapsulant 901. Itshould be noted that FIGS. 10 and 11 are not to scale and that the firstlayer of encapsulant 901 is preferably very thin (i.e., preferably onlya few microns). The encapsulant 901 is deposited essentially over theentire front side of the substrate 405 in order to prepare the surfaceof the substrate to receive a polymer, as will be discussed in moredetail below. In accordance with the preferred embodiment of the presentinvention, the encapsulant 901 is deposited through a screen using aconventional thick film technique. The screen preferably has a densityof 250 wires per inch (with a wire diameter of approximately 0.0016),and an emulsion thickness of 0.0007 inches. The screen masks theencapsulant 901 from forming over the thermistor 409 and metallic layers704, 706 of each of the sensors. However, in the preferred embodiment,the distal end 703, 703' of the anode 701, 701' and the entire cathodeconductor 707 are encapsulated, as shown for example in FIG. 7. A highquality encapsulant is preferably used which will not undergo chemicalalteration in the presence of a caustic solution (such as blood or otheraqueous solvents). For example, in the preferred embodiment, theencapsulant is part number ESL 4904, available from Electro-ScienceLaboratories, Inc. However, the thermistor 409 is preferably notencapsulated with the higher quality encapsulant, since such highquality encapsulants typically require firing at high temperatures (850°C., for example in the case of encapsulant used in the preferredembodiment). Such high temperatures will cause the thermistor 409 todeform. Therefore, only after firing the high quality encapsulant canthe thermistor be encapsulated. Accordingly, in the preferred embodimentof the present invention, the thermistor 409 is encapsulated with anencapsulant which may be fired at a low temperature.

In the preferred embodiment of the present invention, a second layer ofencapsulant 905 is deposited only over the cathode conductor 707 inorder to ensure that the cathode conductor is securely isolated. In oneembodiment of the present invention, the second layer of encapsulant 905is applied in two screening procedures in order to provide a totaldesired thickness for both the first and second layers of encapsulant ofapproximately 27-47 μM. While alternative embodiments of the presentinvention may employ an encapsulant layer which differs in thickness, athickness in the range of approximately 27-47 μM provides satisfactoryisolation of the cathode conductor 707. Furthermore, a single layer ofencapsulant provides sufficient treatment of the surface of thesubstrate 405 to allow a polymer to be deposited and bonded to thesubstrate 405, as further explained below.

After the encapsulant 901, 905 are deposited over the cathode conductor707, a hole is preferably laser drilled through the encapsulant 901, 905to expose a portion of the cathode conductor 707, and thus form thecathode 717. The cathode may be laser drilled either before or afterfiring the encapsulant. The laser targets 417, 418 are used to visuallyalign the laser apparatus in order to drill the hole at the correctlocation. That is, the lower horizontal edge of the target 417identifies a line in the horizontal direction. Likewise, the leftmostedge of the laser target 418 identifies a line in the verticaldimension. The cathode is then formed at the intersection of these twolines. Alternatively, the cathode 717 is formed by masking a portion ofthe cathode conductor 707 in order to prevent the encapsulant 901 fromforming over that portion of the cathode conductor 707. In yet anotherembodiment of the present invention, the cathode 717 may be exposed by achemical etch. It will be clear to those skilled in the art thatnumerous other methods may be used to expose a portion of the cathodeconductor 707 in order to form a cathode 717.

After applying the first and second encapsulant layers to the front ofthe substrate 405, a thermistor encapsulant 413 is deposited over thethermistor 409. The thermistor encapsulant 413 can be fired at arelatively lower temperature (such as approximately 595° C.) and thusfiring of the thermistor encapsulant 913 does not disturb the geometryof the thermistor 409. In one embodiment of the present invention, thethermistor encapsulant 413 is applied in two screenings in order toachieve a desired thickness and to ensure that no pores are formed inthe encapsulant 413. It will be understood by those skilled in the artthat the encapsulant over the thermistor 409 should remain relativelythin in order to avoid adding any delay in the sensing of thetemperature of the sensor assembly 400. In addition, a resistorencapsulant 415 is deposited over the resistor 412 on the back side ofthe substrate 405. The resistor encapsulant 415 is preferably the samematerial as the thermistor encapsulant 413.

After the resistor encapsulant 413 has been deposited on the back sideof the substrate 405, a first polymer layer 1101 is deposited on thefront side of the substrate 405. The first polymer layer (together withthe first encapsulation layer 901) forms the lower wall 902 of aplurality of sensor cavities 903 (see FIGS. 10 and 11). The polymer ofthe preferred embodiment of the present invention is screen printable,absorbs minimal moisture, chemically isolates the membrane chemistriesof adjacent cavities, and produces a strong solution bond with thepolymeric membrane. The polymer also forms a strong bond with thedielectric layers when exposed at the inside surface of the cavity by anappropriate solvent (such as tetrahydrofuran, Xylene, or anycyclohexanone solvent) in the membrane formation, as will be discussedin further detail below.

The polymer used to form the layer 1101 is preferably a composition of28.1% resin acrylic, 36.4% carbitol acetate, 34.3% calcined kaolin, 0.2%fumed silica, and 1.0% silane, noted in percentage by weight. Theacrylic resin is preferably a low molecular weightpolyethylmethacrylate, such as Elvacite, part number 2041, availablefrom DuPont. The calcined kaolin is preferably a silaninized kaolin,such as part number HF900, available from Engelhard. The silane ispreferably an epoxy silane, such as trimethoxysilane. Silane bonds tothe hydroxyl groups on the glass encapsulant over the substrate, and yetis left with a free functional group to crosslink with the resin'sfunctional group. In accordance with one embodiment of the presentinvention, the first polymer layer 1101 is deposited in three screeningprocesses in order to attain the desired thickness (i.e., preferablyapproximately 0.0020 inches). The first polymer layer is dried aftereach screening process. A second polymer layer 1103 is deposited to forman upper wall 904 of the sensor cavities 903. The first and secondpolymer layer 1101, 1103 differ only in the diameter across the cavityat the lower cavity wall 902 and at the upper cavity wall 904 and thenumber of screening processes that are required to achieve the desireddepth. In the case of the second polymer layer, 10 screening proceduresare performed. The second polymer layer is dried after each screeningprocedure. In addition, after the last two procedures, the polymer isboth screened and cured. In the preferred embodiment of the presentinvention, the last screening procedure may be omitted if the secondpolymer layer has achieved the desired thickness (i.e., preferably0.0075-0.0105 inches after curing).

The diameter of the cavities are preferably carefully controlled to aidin controlling the deposition of the membranes which are placed over theelectrodes of the sensors (i.e., the shape and thickness of themembranes). That is, the sensor cavities enable a droplet of polymericmembrane solution to be captured and formed into a centrosymmetric formover the electrode with sufficient surface contact with the walls of thecavity to assure that the membrane remains physically attached.

Preferably, the sensor cavities 903 for the pH sensor 403e, theelectrolyte sensors 403f, 403g, 403h, and the hematocrit sensor 403c,403d, each have a total depth of approximately y=0.0075 inches, adiameter at the upper wall 904 of approximately x₁ =0.070 inches, and atthe lower wall of approximately x₂ =0.06 inches (see FIG. 10). Thediameter x₃ of the carbon dioxide sensor cavity 903 is slightly largerthan the diameter x₁ of the electrolyte sensors 403e-403f and thehematocrit sensor electrodes 403b, 403c. In the preferred embodiment,the diameter x₃ is equal to 0.078 inches (see FIG. 11). It should beunderstood that a membrane of the same thickness may be produced byincreasing the diameter of the sensor cavity 903 and increasing thevolumetric quantity of the membrane solution that is applied to thesensor in proportion to the increase in the volume of the cavity.Likewise, the same thickness can be maintained by decreasing thediameter of the sensor cavity 903 and proportionally decreasing thevolumetric quantity of the membrane solution. It will be clear to thoseskilled in the art that in an alternative embodiment of the presentinvention, the sensor cavities may have a shape other than the generallycylindrical shape disclosed above. For example, in accordance with oneembodiment of the present invention, the electrodes are formed in anoval shape to reduce the required volume of a sample. However, in thepreferred embodiment, the sensor cavities are either cylindrical orgenerally conical.

Once the sensor cavities 903 have been formed and the polymer layersdried, the surface of each silver potentiometric electrode is chemicallychloridized. The cavity 903 of each ion sensitive sensor is filled withan electrolyte which is appropriate to the particular type of sensor403.

All of the aforementioned electrolytes are preferably encapsulated by aselectively permeable, hydrophobic membrane that serves to trap theelectrolyte against the electrode. Such membranes include a polymer, aplasticizer, an ionophore, a charge screening compound (also known as aphase transfer catalyst), and a solvent. The membranes are selectivepermeable barriers that restrict the free passage of all but the desiredion. The membrane preferably comprises an inert lypophilic polymerdispersed in an organic plasticizer.

All membrane solutions are dispensed in the sensor cavities usingautomated fluid dispensing systems. These systems have three main parts:(1) a horizontal x-y-z motorized and programmable table (such as thoseavailable from Asymtek of Carlsbad, Calif.); (2) a precision fluidmetering pump (such as those available from Fluid Metering, Inc. ofOyster Bay, N.Y.); and (3) a personal computer control unit. All threeparts are linked by a digital communication protocol. Software forset-up and dispensing a sequence of liquid microvolumes communicates thex, y, and z positions to the table, and timing of the dispensing pumpcontroller. At each cavity, the metering pump transfers a preset volumeof electrolyte or membrane solution through fine diameter tubing from asupply reservoir to a needle or nozzle mounted on the motorized axes ofthe table and then to the substrate cavity. The fluid may besuccessfully dispensed with a number of different pumps; pinch tube,rotary positive displacement or diaphragm valves. The drop size isgenerally no larger than one diameter of the sensor cavity.

After dispensing the aqueous or organic solution, the membrane is formedby drying or curing liquid. Drying removes the solvent components byevaporation. The drying process may be performed by heating or applyinga vacuum pressure. Some organic solutions may be cured either thermallyor by exposure to ultra-violet radiation.

The combination of the geometry, membrane composition, and aqueous ororganic internal electrolyte have been found to yield membranes ofminimal thickness, with controlled diffusion paths so thatpotentiometric sensors may detect a varying concentration of gas.Elimination of in-plane electrical connections to the electrode by useof a subminiature through hole assures better control of theelectro-chemical processes. In addition, the use of subminiature throughholes improves the flatness of the bonding surface of the polymercoating laminated on the substrate bonding and sealing of the flowcell.

FIG. 12 is a top plan view of the sensor assembly 400 installed within apreferably transparent or translucent plastic encasement 1200. FIG. 13is a cross-sectional view of the sensor assembly 400 installed in theplastic encasement 1200. In accordance with one embodiment of thepresent invention, the encasement 1200 is a transparent plastic havingan outside dimension of less than 0.5 inches by 2.0 inches by 0.25inches. After each of the electrodes have been completed, the pads 411are plated with solder. The solder provides an electrical and mechanicalinterface between the pads 411 and contacts 1209 of a conventionalelectrical surface mount connector 1205. The contacts 1209 of thesurface mount connector 1205 are soldered to the pads 411 in aconventional manner. In addition, the connector 1205 is preferablysecured to the substrate 405 by an adhesive, such as an epoxy glue.Electrically conductive pins 1207 of the conventional connector 1205permit the sensor assembly 400 to be easily installed and in, andremoved from, a blood analyzer (not shown). Use of a conventionalsurface mount connector 1205 results in a reliable interface to theblood analyzer instrumentation, provides a simple design, low costconstruction, a simple test interface, and allows critical connectionsto be spaced apart to ensure high electrical resistance between eachcritical connection. Furthermore, the conventional surface mountconnector 1205 allows the present invention to be mass produced at lowcost, and makes the present invention analogous to familiarsemiconductor dual-in-line packages.

The front side of the sensor assembly 400 is enclosed in the plasticencasement 1200 which forms a flowcell 1201 and a reference cell 1203. Alap joint 1211 is preferably formed between the sensor assembly 400 andthe encasement 1200. In accordance with the preferred embodiment of thepresent invention, an adhesive, such as epoxy glue, is used to securethe sensor assembly 400 in the encasement 1200. The encasement 1200 isformed with inlet and output ports 1202, 1204, respectively. The inletand outlet ports 1202, 1204 allow a sample to be injected into, anddischarged from, the flowcell 1201. The adhesive seals the referencecell 1203 and the flowcell 1201 along the lap joint, such that fluid canonly enter and exit through the inlet and outlet ports 1202, 1204. Theencasement is preferably formed of a material having low oxygenpermeability, low moisture permeability, which is transmissive toultraviolet radiation, and which is resistant to color change uponexposure to ultraviolet radiation, such as a composition of acrylic,styrene, and butadine. Because even the preferred composition absorbsmoisture, the encasement 1200 is preferably formed with a third cell1213. The third cell 1213 reduces the amount of encasing material whichis adjacent to the flowcell 1201. However, it will be clear to thoseskilled in the art that such a third cell 1213 is not necessary for theproper operation of the present invention. In addition, in oneembodiment of the present invention, the amount of encasing material isreduced to a minimum to reduce the absorption of oxygen from a samplewhich is present in the flowcell 1201.

The flowcell 1201 is formed to ensure that a sample which enters theflowcell comes into contact with each of the sensors 403. Furthermore,the flowcell 1201 is very shallow, thus the volume of the flowcell 1201is very small (i.e., 0.05 milliliters in the preferred embodiment). Avery thin reference channel 1206 (preferably 0.005-0.010 inches indiameter) between the reference cell 1203 to the flowcell 1201 providesionic contact between the reference medium which resides within thereference cell 1203. The reference medium may be any well knownreference electrolyte in solution or gel form. However, in the preferredembodiment, the reference medium is preferably a natural polysaccharide,such as agarose, gelatin, or polyacrylamide. The greater viscosity ofthe reference medium used in the preferred embodiment retardsevaporation of the reference medium, as well as preventing the referencemedium from intermingling with the fluids in the flowcell 1201. Thereference medium is preferably introduced into the reference cell 1203after the sensor assembly 400 is installed in the encasement 1200. Inaccordance with the present invention, a vacuum is created in theflowcell 1201 and the reference cell 1203 by applying a low pressuresource to either the inlet or outlet port 1204, 1206. The referencemedium is then applied to the other port 1206, 1204. Preferably, thereference medium is heated to approximately 37°-50° C. by the heater 601or by application of heat through an external heat source to reduce theviscosity of the reference medium, and thus allow the reference mediumto completely fill the reference cell 1203. Once the gel has filled thereference cell 1203, any excess reference medium is gently flushed fromthe flow channel prior to allowing the reference medium to cool. In analternative embodiment of the present invention, the viscosity of thereference medium may be increased in response to a chemical reactionbetween the medium and a catalyst which is placed into the referencechannel either before or after the reference medium.

It should be noted that when the height of the fluid column over thesensor array has been minimized to conserve sample volume (0.10 inches,for example), measurement is preferably made within 10-15 seconds afterthe sample has entered the flowcell 1201.

It will be seen from the above description of the present invention,that the sensors are not separable into parts, but rather form a signalmodular unit, designed for a predefined life, installed once, and thendiscarded. Discarding the unit is economically feasible due to the lowcost at which such sensor assemblies can be fabricated. The presentinvention makes it possible to provide a low cost system which is builtaround standardized electronic assemblies by providing a low cost, massproducible sensor assembly that has highly accurate and reproducibleresults.

It should be clear to those skilled in the art that the use ofsubminiature through holes to route electrical signals from theelectrodes of the sensors to the opposite side of the substrate allow achemically selective membrane overlaying the planar electrode tofunction with the desired sensor reaction mechanism while providing ameans for packing a number of sensors into a relatively small area onthe surface of the substrate. The use of the subminiature through holesalso allows for excellent physical isolation of the sample from theconductors that carry the electrical signals between the sensorelectrodes and the instrumentation used to process those signals. Thisphysical isolation results in very high electrical isolation betweensignals generated by each of the sensors.

FIG. 14a-14c illustrate three alternative embodiments of the presentinvention in which the relative positions of the sensors differ fromthose shown in FIG. 2.

Summary

A number of embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention. Forexample, while the present invention is described generally as beingfabricated using a thick film technique, any other well known layeredcircuit technique may be used, such as thin film, plating pressurizedlaminating, and photolithographic etching. Furthermore, substrates for anumber of sensor assemblies may be fabricated concurrently on a singlesection of ceramic material which has preferably been scored (orscribed) to allow for easy separation into individual substrates afterdeposition of all of the components of the sensor assembly, and prior toinstallation in an encasement. Accordingly, it is to be understood thatthe invention is not to be limited by the specific illustratedembodiment, but only by the scope of the appended claims.

What is claimed is:
 1. A method for drilling subminiature through holesthrough a substrate of a sensor assembly, including the stepsof:providing a polycrystalline ceramic substrate capable of withstandinga of 6 to 9 for a period of at least a week; providing a laser;energizing the laser and generating a laser beam; energizing the lasersuch that the laser beam focuses at a predetermined position located onthe substrate until a through hole is formed; and annealing thesubstrate at a sufficient temperature and duration to oxidize andcrystallize the surface area of the hole.
 2. The method of claim 1,wherein the laser is a pulsed in a range of 200 to 1000 Hz with a pulselength in the range of approximately 38 to 400 μsec. and a totalduration of in the range of approximately 0.25 to 0.50 seconds per hole.3. The method of claim 2, further comprising the step of blowing airacross the location on the surface of the substrate at which the laseris focused at a pressure of at least approximately 20 pounds per squareinch.
 4. The method of claim 3 further including the step of coating thesubstrate with a water soluble coating before energizing the laser. 5.The method of claim 4, wherein the water soluble coating is acrylicresin.
 6. The method of claim 4, wherein the water soluble coating ispolyvinyl alcohol.
 7. The method of claim 4, wherein the step of coatingfurther includes dipping the substrate in a solution of acrylic resin.8. The method of claim 4, wherein the substrate is alumina.
 9. Themethod of claim 8 wherein the alumina substrate is approximately 96%pure.
 10. The method of claim 8 wherein the alumina substrate isapproximately 99% pure.
 11. The method of claim 8, wherein the laser isa sealed CO₂ laser.
 12. The method of claim 1, wherein the substrate isalumina.
 13. The method of claim 12, wherein the substrate is annealedat a temperature in the range of 1000°-1400° C.
 14. The method of claim13, wherein the substrate is annealed at a temperature in the range of1100°-1200° C.
 15. The method of claim 14, wherein the step of annealingfurther includes the step of increasing and decreasing the temperatureat a rate of 1° C. per minute.
 16. The method of claim 15 wherein thesubstrate is annealed for approximately three hours.
 17. A method fordrilling subminiature through holes through a substrate of a sensorassembly, including the steps of:providing a polycrystalline ceramicsubstrate capable of withstanding a pH of 6 to 9 for a period of atleast a week; coating the substrate with a water soluble coating;providing a CO₂ laser; energizing the laser and generating a laser beam;positioning the laser such that the laser beam focuses at apredetermined position located on the substrate and operating the laserat an energy level sufficient that a through hole is formed; andannealing the substrate at a temperature in the range of 1000°-1400° C.for a sufficient duration to oxidize and crystallize the surface area ofthe hole.
 18. The method of claim 17, wherein the laser is a pulsed in arange of 200 to 1000 Hz with a pulse length in the range ofapproximately 38 to 400 μsec. and a total duration of in the range ofapproximately 0.25 to 0.50 seconds per hole.
 19. The method of claim 18,wherein the step of annealing the substrate includes annealing at atemperature in the range of 1100°-1200° C.
 20. The method of claim 18,wherein the step of coating includes dipping the substrate in a solutionof polyvinyl alcohol.
 21. The method of claim 20 wherein the step ofproviding the substrate includes providing a substrate that isapproximately 96% pure alumina.
 22. The method of claim 21 wherein thestep of providing the substrate includes providing a substrate that isapproximately 0.025 to approximately 0.040 inches in thickness.
 23. Themethod of claim 22 wherein the step of forming the through hole includesforming the through hole to have a diameter that is approximately 0.002to approximately 0.006 inches.
 24. A method for drilling subminiaturethrough holes through a substrate of a sensor assembly, including thesteps of:providing an alumina substrate having a thickness that isapproximately 0.025 to approximately 0.040 inches in thickness; coatingthe substrate with a water soluble coating; providing a closed CO₂laser; energizing the laser and generating a laser beam; forming athrough hole in the substrate by the further steps of positioning thelaser such that the laser beam focuses at a predetermined positionlocated on the substrate and operating the laser wherein the laser is apulsed in a range of 200 to 1000 Hz with a pulse length in the range ofapproximately 38 to 400 μsec. and a total duration of in the range ofapproximately 0.25 to 0.50 seconds per hole to thereby form a throughhole that is approximately 0.002 to approximately 0.006 inches indiameter; and annealing the substrate at a temperature in the range of1000°-1400° C. for a sufficient duration to oxidize and crystallize thesurface area of the hole.
 25. The method of claim 24 wherein the aluminasubstrate is approximately 96% pure.